During gas chromatography analysis the elution of analytes of interest is preceded by the elution of vaporised solvent (typically 1 μL) in which the sample was contained. Under typical conditions the solvent will start to elute from the gas chromatography column within the first half a minute of the acquisition and will typically continue to elute for around two minutes.
The solvent front can have a detrimental effect on the stability and reproducibility of the ionisation of subsequent analytes when a gas chromatograph is coupled to an atmospheric pressure corona discharge ionisation source.
An atmospheric pressure corona discharge ionisation source operating in a positive ionisation mode under optimal conditions operates with a coronal form know as a “glow discharge corona”. This type of corona is characterised by a steady current with little or no noise or sparking.
The mechanism for the establishment and sustainment of this corona form under the conditions under which this source operates is as follows. An electron in close proximity to the corona pin tip is accelerated towards the pin due to the high positive potential applied. If the field gradient is sufficiently high then the electron can gain enough energy between collisions with the gas about the corona pin to cause the ionisation of molecules it collides with. This ionisation event will result in a radical cation of the gas molecule and the release of an additional electron. As this ionisation event has occurred within a high field gradient it is probable that the two electrons will gain sufficient energy in their subsequent acceleration towards the corona pin to cause further ionisation. In this fashion a so-called “electron avalanche” is formed. Some of these electrons will undergo recombination reactions with the radical cations of the gas molecules, resulting in the release of a high energy photon. These photons can escape the high potential in the proximity of the tip of the corona pin and subsequently cause ionisation and the release of more electrons further out in the electric field. These electrons are then accelerated towards the pin and initiate further electron avalanches. In this fashion, once a corona has been established it can typically sustain itself.
When using a corona pin in current regulation mode (where the voltage is varied to maintain a requested current), the voltage applied to the pin is seen to increase significantly in the presence of the solvent. Once the solvent front has passed, the voltage applied to the pin in order to maintain the required corona current can vary significantly from one acquisition to the next with the result that a corresponding variation in analyte response is observed.
US 2007/0181801 (Yamada) discloses a mass spectrometer capable of switching between two different ion sources in a single experimental run. In US 2007/0181801 (Yamada), a sample is added to the upstream of a gas chromatography (“GC”) column, and an atmospheric pressure chemical ionisation (“APCI”) mass spectrum is obtained for an analyte elution peak. If a measured peak corresponds to an unknown ingredient of the sample (e.g. based on a known database), then the APCI ion source is switched OFF and instead an electron ionisation (“EI”) ion source is used in order to obtain an EI spectrum for a peak of the unknown ingredient.
US 2005/0035286 (Micromass) discloses a mass spectrometer having an APCI ion source connected with a GC separation device. The current applied to a corona needle is repeatedly varied between two or more settings during a single experimental run in order to ionise a mixture containing both low and highly polar analytes.
WO 2014/021960 (Verenchikov) discloses a GC separator combined with an ion source, such as a chemical ionization (CI) source. The ion source has the capability to switch between ionization polarity by reverting the potential on ionizing corona discharge.
The problems inherent with coupling a gas chromatograph to an atmospheric pressure chemical ionisation ion source and operating the ion source in a conventional manner will be described with reference to FIGS. 1-3.
FIG. 1 shows an example chromatogram (total ion current “TIC”) acquired on an Atmospheric Pressure Chemical Ionisation (“APCI”) mass spectrometer coupled to a Gas Chromatograph (“GC”). The solvent front can be seen as a significant rise in the total ion current between 0.7 and 1.5 minutes from the start of the analysis.
Throughout the entire analysis the corona pin was set to regulate at 2.0 μA with the voltage applied to the corona pin being adjusted in order to achieve this.
FIG. 2 shows the voltage that was applied to the corona pin in order to maintain or regulate the corona current at 2.0 μA for seven successive acquisitions under the same conditions.
The influence of the solvent front on the voltage which has to be applied to the corona pin in order to maintain the corona current steady at 2.0 μA can clearly be seen in the data shown in FIG. 2.
It is apparent from FIG. 2 that the voltage applied to the corona pin rises as the vaporised solvent is introduced within the ionisation region around the tip of the corona pin and disrupts the corona discharge process. As the solvent vapour is gradually depleted the voltage is seen to drop towards the value at which it was regulating before the solvent front. However, the exact value of the corona pin voltage at any time during the acquisition is markedly different from one acquisition to the next.
FIG. 3 demonstrates how the variation in corona pin voltage can adversely influence the analyte response of 4-bromophenyl phenyl ether and shows how the response across seven sequential acquisitions was highly variable.
The analyte shown in FIG. 3, namely 4-bromophenyl phenyl ether, elutes from the gas chromatograph at an acquisition time of 6.54 mins. FIG. 3 shows the peak response for the seven successive acquisitions for which corona voltage data is shown in FIG. 2.
The corona voltage at the elution time (6.54 mins) of this particular analyte is also plotted in FIG. 3 using the secondary axis. There is a clear inverse correlation between the corona voltage and the analyte response.
It is desired to provide an improved mass spectrometer.